1. Field of the Invention
The present invention relates generally to image forming apparatus, and more particularly to a color MFP (Multi-Function Peripheral) and a monochromatic MFP (Multi-Function Peripheral) having a color scan function.
2. Description of the Related Art
In recent years, an apparatus called a multi-function peripheral (MFP), in which functions of a copier, a facsimile and a printer are integrated, has widely been used.
A color MFP has a function of executing scan, copy and print in color.
In a conventional color MFP, in general, when a monochromatic (achromatic) original is to be copied, the original is read by a color scanner and the read image information on the original is sent to a printer section, thereby forming an image. In addition, in a case where the color MFP is provided with a monochromatic copy mode or a single-color copy mode, monochromatic image information (luminance information) is, in general, extracted from color image information that is read by the color scanner, and the printer is operated in the monochromatic print mode or single-color print mode, thereby obtaining a monochromatic image or a single-color image.
The color MFP is generally constructed to include a scanner section, in which a color sensor is incorporated, and a 4-rotation-type color printer (to be described later).
A monochromatic MFP with a color scan function has such a function that copy and print are executed in a monochromatic mode and scan is executed in a color mode.
Like the color MFP, in the monochromatic MFP with the color scan function, when a monochromatic original is to be copied, the original is, in general, read by a color scanner and monochromatic image information (luminance information) is extracted from the read color image information, and the printer is operated in the monochromatic print mode, thereby obtaining a monochromatic image.
The monochromatic MFP with the color scan function is generally constructed to include a scanner section, in which a color sensor is incorporated, and a monochromatic printer.
In addition, a CCD sensor that is used in the MFP, that is, a CCD sensor functioning as a color sensor that is incorporated in the scanner for reading a color image, is generally composed of three lines of, e.g. R, G and B (R: red, G: green, and B: blue), which have sensitivities for only specific light wavelengths).
However, in the case where an original that is read by color scanning is copied as a monochromatic document by a conventional method, such problems as a decrease in image quality and deficiency in image output speed have been pointed out.
In the case where monochromatic image information is to be obtained by the above-described sensor, the monochromatic image information is compounded from R, G and B color information, which is separately read. Thus, if lens characteristics have chromatic aberration or if non-uniformity in speed (jitter) occurs in the driving system at a time of reading, misregistration would occur in images that are captured by the R, G and B sensors. Consequently, an edge portion, in particular, of an image, which should normally be a monochromatic (achromatic) image, may be captured as a color image, or an image with color misregistration (chromatic aberration), which is larger than a real image, may be captured. A monochromatic image, which is obtained on the basis of such information, becomes different from a normal image, and the image quality deteriorates.
FIGS. 34, 35, 36 and 37 explain this phenomenon. For example, if chromatic aberration or driving jitter occurs when a rectangular original image (achromatic) with no density gradient is read by three RGB line sensors, images that are read by the RGB line sensors are misregistered in a main scan direction and a sub-scan direction. If such image information is output from a printer section, the shape of the output image is distorted, as shown in the Figures. Consequently, density gradient occurs at the edge portions. Thus, the quality of the image that is output from the printer deteriorates (problem of image quality).
In addition, the three RGB line sensors that are built in the scanner section are normally provided with color filters for passing only light of specific wavelength bands. The sensitivity of these line sensors is lower than that of a sensor with no such filter. As a result, compared to an apparatus that is dedicated to a monochromatic mode, there is a problem that the speed of monochromatic copy is low. If the speed is to be increased, it is necessary to increase the size of the sensors, thereby enhancing the sensitivity, or to provide a large light source. This leads to an increase in size of the apparatus (problem of reading speed, and problem of increase in size of the scanner section).
Furthermore, in the case where the print output speed of the color printer section differs between the color mode and monochromatic mode, an unmatched relation in speed occurs between the scanner section and the printer section. Consequently, the performance of the printer section cannot fully be exhibited. Besides, this leads to such problems that the lifetime of the printer section decreases, and the lifetime of consumable components of the printer section, such as a photoconductor drum, becomes shorter than the normal lifetime (problems of apparatus lifetime and consumable component lifetime).